426 



THE CHANGING GENERATIONS 



Fishes. The little ostracoderms persisted for a time, only to be crowded 

 out by their descendants. Among the newcomers were many strange 

 types that did not long endure, but others were the ancestors of our 

 modern sharks and bony fishes. All had jaws and fed on solid food. 



The -placoderms were the most primitive. They included a heterogeneous lot of 

 bizarre forms, a step above the ostracoderms in organization but not very closely 



Fig. 27.7. Life of a Devonian coral reef, showing cripoids or sea lilies, solitary corals, colonial 

 corals, cephalopods, trilobites, and other organisms. {Courtesy University of Michigan 

 Museums.) 



Fig. 27.8. The Devonian "spiny shark" or acanthodian, Climatius. {Redrawn from Romer, 

 by permission The Williams & Wilkins Company.) 



related among themselves. All possessed a well-developed bony armor of plates, 

 or enameled scales, and had primitive jaws, the upper jaw being fastened directly 

 to the brain case. The tail tapered to a point, which was usually bent upward 

 with a tail fin projecting from its lower surface. The group included the acantho- 

 dians, the joint-necked fishes, and the strange antiarchs, as well as other oddities. 

 The acanthodians were small fresh-water fishes with numerous fins, and with a 

 complete armor of bony scales. The fins were supported by stiff, immovable spines 

 which are common fossils in Devonian rocks. Although these fishes are often 

 called "the spiny sharks," they were not shark ancestors, as was once supposed, 



